Custom (or bespoke) coachwork is not to be confused with a "custom car", a distinct genre of automobile modification. Coachwork bodied vehicles employ a rolling chassis to avoid the vast expense of designing and building a suitable unibody or monocoque structure. While the enormous cost of suitable machinery to make steel structures may be avoided by moulding synthetic materials for one-off bodies the high costs of structural design and development remain prohibitively expensive.

As well as true bespoke bodies, coachbuilders also made short runs of more-or-less identical bodies to the order of dealers or the manufacturer of a chassis. The same body design might then be adjusted to suit different brands of chassis. Examples include Salmons & Sons' Tickford bodies with a patent device to raise or lower a convertible's roof, used on their 19th century carriages, or Wingham convertible bodies by Martin Walter.

The term coach-built originally implied a body with a wooden frame, but has been expanded to describe purpose-built vehicles such as an RV or motorhome bodied upon a rolling chassis. This approach is distinct from a conversion built inside an existing vehicle body.

In the early days of motoring days only a few manufacturers supplied their own bodies.
Commonly, acquiring a new vehicle commonly required the selection of both a vehicle manufacturer - to provide a rolling chassis - and a coachbuilder to build and mount a body upon it.

The maker would provide the coachworks with a the chassis frame, drivetrain (consisting of an engine, gearbox, differential, axles, and wheels), brakes, suspension, steering system, lighting system, spare wheel(s), front and rear mudguards and (later) bumpers, scuttle (firewall) and dashboard. The very easily damaged honeycomb radiator, later enclosed and protected by a shell, typically became the main visual element identifying the chassis' brand. To maintain some level of control over the final product, chassis manufacturers' warranties would be voided by mating them with unapproved bodies.

When popular automobile manufacturers brought body building in-house, larger dealers or distributors of ultra-luxury cars would commonly pre-order stock chassis and the bodies they thought most likely to sell, and inventory them in suitable quantities for sale off their showroom floor. In time, the practice of commissioning bespoke coachwork dwindled to a prerogative of wealth.

The practice remained in limited force after World War II, with both luxury chassis and high-performance sports cars and gran turismos, waning dramatically by the late 1960s. Even Rolls-Royce acquiesced, debuting its first unibody model, the Silver Shadow, in 1965, before taking all R-R and Bentley bodying in-house.

Independent coachbuilders survived for a time after the mid-20th century, making bodies for the chassis produced by low-production companies such as Rolls-Royce, Ferrari, and Bentley.[3] Producing body dies is extremely expensive (a single door die can run to US$40,000), which is usually only considered practical when large numbers are involved—though that was the path taken by Rolls-Royce and Bentley after 1945 for their own in-house production. Because dies for pressing metal panels are so costly, from the mid 20th century, many vehicles, most notably the Chevrolet Corvette, were clothed with large panels of fiberglass reinforced resin, which only require inexpensive molds. Glass has since been replaced by more sophisticated materials, if necessary hand-formed. Generally these replace metal only where weight is of paramount importance.

The advent of unibody construction, where the car body is unified with, and structurally integral to the chassis, made custom coachbuilding uneconomic. Many coachbuilders closed down, were bought by manufacturers or changed their core business to other activities:

^Construction has always been a skilled trade requiring a relatively lightweight product with sufficient strength. The manufacture of necessarily fragile, but satisfactory wheels by a separate trade, a wheelwright, held together by iron or steel tyres, was always most critical. From about AD 1000 rough vehicle construction was carried out by a wainwright, a wagon-builder. Later names include cartwright (a carpenter who makes carts, from 1587); coachwright; and coachmaker (from 1599). Subtrades include wheelwright, coachjoiner, etc. The word coachbuilder first appeared in 1794. Oxford English Dictionary 2011